@asalcedo2f63dd2: "The main flow and return 35mm pipes in the central heating of my home
are connected by a 22mm By-Pass with a gate valve. This by-pass is near
the boilers before any control valves.
I read that the purpose of the By-Pass is to prevent local boiling in
the boiler.
I have two questions, 
1. At the moment the By-Pass valve is fully open, the system seems to
work well, but, wouldn't it be more efficient if it were closed,
assuming no boiling appears?
2. I am replacing the current 10year old Vokera boilers with new
Worcester High Efficiency condensing boilers. For these boilers, is it
better to have the by-pass valve open or closed?
asalcedo"
    @mrace: "<quote>
I had a house with a by-pass because all of the rads had TRVs.  This 
meant that in a worst case scenario all of the TRVs would be coused an 
there would be no way of circulating water around the system while the 
boiler cooled down, leadind to potentioa damage to the boiler.
My present house was set up so that there were TRVs on all rads except 
the bathroom, so this was used as a by-pass.  It also meant that the 
bathroom was always very hot when the heating was on.  When I had some 
work done on the system I talked to the heating engineer about this and 
he fitted a by-pass with an 'automatic valve'  (sorry, no more info than 
this) an a TRV to the bathroom rad.  This system has worked well for the 
last 4 years.
I would guess that all you need to do is to partially close the gate 
vave to abou a quarter open so that there is always an alternative 
circuit to prevent hot water accumulating in the boiler if all the rads 
are closed
Malcolm"
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